The present invention relates to transmission apparatus including encoders and decoders useful for forward error correction and more specifically to transmission circuits in which the effects of signal loss, noise, and interference, or any of them, are time variable. The present invention is particularly useful in high frequency and troposcatter radio circuits.
Signal fading, impulsive noise and interference are deleterious effects frequently encountered in many transmission circuits. Where these effects are present, the quality of transmission is deteriorated and an increase in the transmission error-rate occurs. Radio circuits employed to transmit teleprinter information are particularly susceptible to these deliterious effects. Radio teleprinter information is typically transmitted at a rate of up to 75 bits per second (baud).
The quality of a transmission circuit is frequently measured in terms of its character error rate (CER). The character error rate is defined as the percentage of erroneous characters received relative to the total number of characters transmitted in a given time period.
Prior art techniques for improving the performance of data transmission circuits have utilized many different forms of redundancy in connection with forward error correction. Diversity systems employing space diversity, polarization diversity, frequency diversity or time diversity have all been known in one form or another.
The term "time diversity" has usually been interpreted to mean synchronous transmission of data two or more times with a time delay between each transmission. Each received data bit is compared with a corresponding delayed data bit. In such systems, synchronous operation is required in order to identify each bit. Synchronous operation has the undersireable requirement of being dependent upon the transmission data rate. A change in data rate requires a corresponding change in synchronous clocking in the transmitter and receiver apparatus. If a difference is observed between bits as a result of a comparison of bits, an error is identified. When an error is identified, one of the data bits, for example the earlier transmitted data bit, is the one selected for actual use. A time diversity system of this type has been described by L. E. Zegers in an article entitled "Error Control in Telephone Channels By Means of Time Diversity" appearing in the Philips Research Report, volumn 22, June, 1967.
The term "time diversity" has also been applied to systems in which data bits are divided in time, with one half of each bit being transmitted on one frequency and the other one half of each bit being transmitted on another frequency.
The performance quality of known diversity systems is not entirely satisfactory particularly when the effects of signal loss, noise or interference have a duration of up to several seconds. There is a need, therefore, for improved and economical apparatus useful in improving transmission and reducing character error rates in transmission circuits.